In automotive manufacturing, polymeric composites are being used increasingly due to their favorable characteristics, including being lightweight, highly-conformable or shapeable, strong, and durable. Some composites are further colorable and can be finished to have most any desired texture.
The increased use in automobiles includes, for instance, in instrument and door panels, lamps, air ducts, steering wheels, upholstery, truck beds or other vehicle storage compartments, upholstery, external parts, and even engine components. Regarding engine components, and other under-the-hood (or, UTH) applications, for instance, polymers are configured, and being developed continuously, that can withstand a hot and/or chemically aggressive environment. Regarding external parts, such as fenders, polymers are being developed that are online paintability and have high heat and chemical resistance over longer periods of time. And many other potential usages in automotive applications are being considered continuously.
With this trend, finding ways to efficiently and effectively join polymer components is becoming progressively important. Compression molding and post-mold joining techniques—e.g., ultrasonic welding—are being used more commonly.
Traditional techniques have various shortcomings. With reference to the figures, and more particularly the first figure, FIG. 1 shows schematically a conventional ultrasonic welding arrangement 100 including a welding horn 102 and two workpieces 104, 106 prior to welding.
In the illustrated step, the horn 102 is lowered to contact a top workpiece 104 of the two. Once in contact with the piece 104, an ultrasonic generator connected to the horn excites high-frequency ultrasonic vibrations, which are passed through the horn to the piece. At the interface heat is generated and the workpiece 104 begins to melt 200 as shown in FIG. 2.
FIG. 3 shows the arrangement 100 after it has been melted significantly so that now-molten material of the workpieces connects the pieces 104, 106.
The technique has shortcomings including transferring material 400 from one or both workpieces 104, 106 remaining on the horn undesirably when the horn 102 is withdrawn, as shown in FIG. 4. The remnant 400 limits horn performance going forward and horn life.
It is contemplated that the horn 102 could be held in the position of FIG. 3, in contact with the workpieces 104, 106 for a prolonged time (e.g., about 12 to 20 seconds in some implementations), allowing the new weld to cool before horn retrieval. Some material 400, though, may still transfer to the horn 102. And cycle time is lengthened undesirably, increasing the cost of production.
According to one alternative, a new horn can be used for each welding. This approach would be cost prohibitive, and the part switching, time consuming.
Another alternative is conventional mechanical fastening. The workpieces can be screwed together, or connected by nuts and bolts, for instance. These connections have shortcomings including unwanted added weight, unsightly exposed portions of the fasteners, and possibly less-robust joints.
Moreover, because some of these materials have relatively-low melting points and low electrical conductivities, there are challenges in efforts to melt the workpieces at an interface between them efficiently, quickly, and with minimal melting of other portions of the workpieces.
Energy directors are sometimes used to expedite and control welding. At least one additional challenge arises, however. Because the energy directors are usually not visible from external at the time for welding, it is difficult for the welder, attempting to focus welding at the director, to determine exactly where that is.
With or without energy directors, have an additional shortcoming of undesirably-high cycle times in addition to the undesirably-high energy requirements. Increased time is cost prohibitive, especially when multiplied by repeated iterations processing in a manufacturing environment—e.g., automobile assembly plant.
When energy directors are used, time is needed to locate the energy directors.
When energy directors are not used, more energy is required to melt a first of the workpieces to a depth reaching the second workpiece in order to join the two. Also, the resulting weld is not focused optimally because the first workpiece melts in a broader area instead of only at and around a target point as it does when a director is used. Although this process does not require locating of energy directors, it does not achieve an ideal weld.
The cycle time in both techniques is further hindered by a need to allow the workpieces to cool before withdrawing the ultrasound horn, and to withdraw the horn very slowly. These costs relate to the fact that heat dissipates slowly through polymers—and so they do not heat or cool rapidly. So, after the ultrasound horn has finished heating as needed for welding, the horn cannot be retrieved until the workpieces are sufficiently cool (e.g., about 12 to 20 seconds in some implementations), else some workpiece surface material will remain attached to the withdrawing horn, affecting the workpiece cosmetically and limiting horn performance and horn life.
Still another shortcoming of both conventional ultrasound welding techniques is that an energy-application surface of a proximate workpiece of the two must be generally flat and square to the energy applicator. A flat surface is needed to ensure sufficient contact by the horn for transmitting sufficient high-frequency (HF) vibration waves to the pieces to create the needed inter-piece heat for welding them together.
For welding via curved, or other non-flat surfaces, a special design horn is required. The resulting process is may not be as robust as desired.
An alternative to these ultrasonic welding techniques for joining polymeric composites is conventional mechanical fastening. The workpieces can be screwed together, or connected by nuts and bolts, for instance. These connections have shortcomings including unwanted added weight, unsightly exposed portions of the fasteners, and possibly less-robust joints.